At temperatures greater than about 1000° C. (1832° F.), high temperature oxidation is the most important form of environmental attack observed with aluminide diffusion coatings. High temperature oxidation is a chemical reaction whose rate controlling process for an aluminide coating is diffusion through a product (oxide) layer. Diffusion is a thermally activated process, and consequently, the diffusion coefficients are exponential functions of temperature. Since the oxidation of aluminide coatings is a diffusion controlled reaction and diffusion coefficients are exponential functions of temperature, the oxidation rate is also an exponential function of temperature. At low temperatures where diffusion coefficients are relatively small, the growth rate of a protective scale on any aluminide coating is also small. Thus, adequate oxidation resistance should be provided by any state of the art aluminide coatings, such as: chromium aluminide, aluminide or two phase [PtAl2+(Ni,Pt)Al] platinum aluminide, all inward grown coatings made by pack cementation. However, at high temperatures where the diffusion coefficients and consequently the oxidation rate increase rapidly with increasing temperature, only coatings which form high purity alumina (Al2O3) scales are likely to provide adequate resistance to environmental degradation.
The presence of platinum in nickel aluminide has been concluded to provide a number of thermodynamic and kinetic effects which promote the formation of a slow growing, high purity protective alumina scale. Consequently, the high temperature oxidation resistance of platinum modified aluminide diffusion coatings generally is better as compared to simple aluminide diffusion coatings not containing platinum.
Many of the problems encountered with the previous industry standard platinum aluminides having a two phase, inwardly grown structure have been overcome by using outwardly grown, single phase platinum aluminide coatings as described, for example, in the Conner et al. technical articles entitled “Evaluation of Simple Aluminide and Platinum Modified Aluminide Coatings on High Pressure Turbine Blades after Factory Engine testing”, Proc. AMSE Int. Conf. of Gas Turbines and Aero Engine Congress Jun. 3-6, 1991 and Jun. 1-4, 1992. For example, the outwardly grown, single phase aluminide diffusion coating microstructure on directionally solidified (DS) Hf-bearing nickel base superalloy substrates was relatively unchanged after factory engine service in contrast to the microstructure of the previous industry standard two phase aluminide coating. Further, the growth of a CVD single phase platinum aluminide coating was relatively insignificant compared to two phase aluminide coatings during factory engine service. Moreover, the “high temperature low activity” outward grown platinum aluminide coatings were observed to be more ductile than inward grown “low temperature high activity” platinum aluminide coatings.
U.S. Pat. Nos. 5,658,614; 5,716,720; 5,856,027; 5,788,823; 5,989,733; 6,129,991; 6,136,451; and 6,291,014 describe a CVD process for forming a single phase, outwardly grown platinum aluminide diffusion coating modified with platinum or other elements on a nickel base superalloy substrate. U.S. Pat. Nos. 5,261,963; 5,264,245; 5,407,704; and 5,462,013 describe typical chemical vapor deposition (CVD) apparatus for forming a diffusion aluminide coating on a substrate.